feat(push): add -o/--output flag (table|json|yaml) to helm push

[jojinkb]

What this PR does / why we need it:

helm push currently only prints a human-readable summary (Pushed: ... / Digest: ...), which makes it awkward to consume the pushed reference and manifest digest programmatically — e.g. to immediately sign the chart with cosign. This PR adds the standard -o/--output flag (table, json, yaml) to helm push, consistent with helm install, helm status, helm get metadata, etc.

$ helm push mychart-0.1.0.tgz oci://registry.example.com/charts -o json
{"ref":"registry.example.com/charts/mychart:0.1.0","digest":"sha256:e5ef611620fb97704d8751c16bab17fedb68883bfb0edc76f78a70e9173f9b55"}

$ helm push mychart-0.1.0.tgz oci://registry.example.com/charts -o yaml
digest: sha256:e5ef611620fb97704d8751c16bab17fedb68883bfb0edc76f78a70e9173f9b55
ref: registry.example.com/charts/mychart:0.1.0

closes #11735

How it works:

The OCI registry client already returns a *registry.PushResult, but OCIPusher discarded it, so the result never reached the CLI. This PR plumbs it through with purely additive functional options, keeping all public Go SDK signatures unchanged per HIP-0004:

• pkg/pusher: new WithPushResultHandler(func(*registry.PushResult)) Option; OCIPusher invokes the handler after a successful push.
• pkg/action: new WithPushResultHandler(...) PushOpt on the push action that forwards the handler to the pusher. Push.Run's signature is unchanged.
• pkg/cmd: helm push binds the shared output flag, captures the result via the handler, and renders it with the existing pkg/cli/output machinery.

Special notes for your reviewer:

• Default (table) output is byte-for-byte identical to today: the registry client keeps writing the Pushed:/Digest: summary (including the underscore warning) directly, exactly as before.
• For json/yaml, the registry client's writer is set to io.Discard so stdout contains only the structured result (this also means the OCI "underscore in ref" warning is not printed in these modes; the ref itself is part of the structured output).
• The structured output is intentionally minimal (ref + manifest digest, mirroring the table summary). More fields from registry.PushResult (chart/prov digests, sizes) can be added later without breaking consumers; happy to include them now if preferred.
• The Pusher interface, uploader.UploadTo, and action.Push.Run signatures are deliberately untouched. feat(push): add --output json/yaml/table flag to helm push #32009 implements the same feature by changing those public signatures; this PR takes a non-breaking approach so it can land within v4.
• The new TestPushCmd exercises the real push path end-to-end against the in-process repotest OCI registry (same infrastructure as TestPullCmd), asserting that -o json/-o yaml stdout parses cleanly and that table output is unchanged.

If applicable:

• this PR contains user facing changes (the docs needed label should be applied if so)
• this PR contains unit tests
• this PR has been tested for backwards compatibility

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This change was developed with AI assistance (Claude Code); I have reviewed and tested it.